1. Field of the Invention
The invention relates to a power supply apparatus suitable for use in a power supply circuit of a switching type.
2. Description of the Related Art
Hitherto, when a cellular phone or a VTR integrated type digital camera (hereinafter, they are referred to as an electronic apparatus) or the like is charged or when the electronic apparatus operates, an AC adapter for supplying a desired power source from a connected commercially available power source to the electronic apparatus is used. The AC adapter has what is called a standby mode in which it is connected to the commercially available power source even if no electronic apparatus is connected. Although the electronic apparatus is not connected, even when a power source is hardly supplied from the AC adapter to the electronic apparatus since the electronic apparatus does not operate, the AC adapter enters the standby mode. Even in the standby mode, a necessary circuit needs to operate in order to switch an operating mode to a supply mode.
In correspondence to the standby mode, the AC adapter has the supply mode for supplying a power source to the electronic apparatus. The supply mode is a mode which is set when the electronic apparatus is connected or the connected electronic apparatus operates.
To reduce an electric power consumption in the standby mode, in case of the switching type power supply circuit, it is necessary to lower a switching frequency and decrease a duty ratio. In the supply mode, on the other hand, it is necessary to raise a switching frequency and increase a duty ratio. For this purpose, a frequency of one oscillator has been switched hitherto.
Since the oscillator is designed so that it can operate even at a high frequency, an impedance of the oscillator decreases. It is, therefore, necessary to also reduce an impedance of a driving circuit for supplying an output of the oscillator to a switching device. There is consequently a problem of an increase in load electric power.
It is, therefore, an object of the invention to provide a power supply apparatus which can suppress an electric power consumption in a standby mode in a switching type power supply circuit.
According to the first aspect of the invention, there is provided a power supply apparatus for outputting a predetermined power source from a commercially available power source, wherein
a primary side of a transformer is provided with first and second oscillators for generating outputs of different frequencies and different duty ratios and a switching unit for switching on the basis of the output of one of the first and second oscillators,
a secondary side of the transformer is provided with a rectifying circuit for rectifying an output of the switching unit and forming an output and detecting means for detecting a standby mode and a supply mode, and
one of the first and second oscillators is selected in accordance with a result of the detection of the detecting means.
The above and other objects and features of the present invention will become apparent from the following detailed description and the appended claims with reference to the accompanying drawings.